1. Technical Field
This invention relates to integrated circuit manufacture and, in particular, manufacture of integrated circuits having shallow junctions.
2. Art Background
In the manufacture of integrated circuits, electrical contact to junctions such as the source and drain of field effect transistors is required. For these devices desirable electrical properties include a junction contact resistance less than 10 ohms and a junction leakage current less than 10.sup.-7 amps/cm.sup.2. Complexities inherent in strict design rules make satisfaction of these requirements significantly more difficult. (The device design rule is the smallest lateral dimension for all features within the device circuit.) For example, since the source and drain junctions are typically no deeper than 0.25 .mu.m at submicrons design rules, any undesirable chemical reaction with the junction induced by the fabrication process quickly destroys it. Additionally, electrical contact is made through an opening in an overlying dielectric to the underlying junction region, e.g., drain or source junction region. Typically, as a consequence of strict design rules, this opening (via) has a high aspect ratio, i.e., greater than 1.1. (Aspect ratio is defined as the thickness of the dielectric at the junction divided by the effective diameter of the via at the junction, i.e., the diameter of a circle having the same area as the via at the junction.) To contact the junction through a high aspect ratio opening requires deposition of a conductive material that conforms to or fills the opening so that the conducting cross-section in the via is adequate to maintain an acceptably low current density and contact resistance. Thus, in summary, to ensure a suitable contact, undesirable chemical reaction with the junction should be avoided while a coating that conforms to or fills the via should be produced.
Generally, it is desirable to utilize aluminum as the electrically conductive contact material because of its high conductivity, etchability, excellent adhesion to silicon oxide, and nominal cost. However, direct deposition of aluminum to produce a suitable coating in a via that has an aspect ratio greater than 1.1 has not been reported. Thus, the simplicity and desirable characteristics of direct aluminum deposition is not available.
Other contact materials have been investigated. For example, the deposition of tungsten by a low pressure chemical vapor deposition (LPCVD) technique has been reported. (See, for example, N. E. Miller and I. Beinglass, Solid State Technol.,25 (12), 85 (1982), E. K. Broadbent and C. L. Ramiller, J. Electrochem. Soc., 131, 1427 (1984), and E. K. Broadbent and W. T. Stacy, Solid State Technol., 49(12), 51 (1985). This technique has the advantage of allowing deposition into vias that have aspect ratios greater than 1.1. However, there is a substantial undesirable interaction between the junction (including a silicide overlying the silicon) and chemical entities introduced during deposition of the tungsten. (See, for example, M. L. Green and R. A. Levy, Semicon East 1985 Technical Proceedings,57 (1985), M. L. Green and R. A. Levy, J. Electrochem. Soc., 132, 1243 (1985), G. E. Georgiou et al, Tungsten and Other Refractory Metals for VLSI Applications II, E. K. Broadbent, Editor, page 225, MRS, Pittsburgh, PA, 1987, and N. Lifshitz et al, Tungsten and Other Refractory Metals for VLSI Applications III, V. A. Wells, Editor, page 225, MRS, Pittsburgh, PA, 1988.) Under certain conditions this interaction is self limiting. That is, the amount of junction silicon consumed during the deposition reaches a maximum value that depends on the reaction conditions. Despite this self-limiting effect, the damage produced is still too extensive for junctions, such as source and drain junctions, shallower than 0.25 .mu.m. Additionally, the adhesion between the deposited tungsten material and the dielectric material, e.g., silicon dioxide, is not entirely desirable. Thus, although tungsten under appropriate conditions produces a conformal coating, this coating still produces a device with unacceptable characteristics.